Purification of Waste-Generated Biogas Mixtures Using Covalent Organic Framework's High CO2 Selectivity.

Development of crystalline porous materials for selective CO2 adsorption and storage is in high demand to boost the carbon capture and storage (CCS) technology. In this regard, we have developed a ß-keto enamine-based covalent organic framework (VM-COF) via the Schiff base polycondensation technique. The as-synthesized VM-COF exhibited excellent thermal and chemical stability along with a very high surface area (1258 m2 g-1) and a high CO2 adsorption capacity (3.58 mmol g-1) at room temperature (298 K). The CO2/CH4 and CO2/H2 selectivities by the IAST method were calculated to be 10.9 and 881.7, respectively, which were further experimentally supported by breakthrough analysis. Moreover, theoretical investigations revealed that the carbonyl-rich sites in a polymeric backbone have higher CO2 binding affinity along with very high binding energy (-39.44 KJ mol-1) compared to other aromatic carbon-rich sites. Intrigued by the best CO2 adsorption capacity and high CO2 selectivity, we have utilized the VM-COF for biogas purification produced by the biofermentation of municipal waste. Compared with the commercially available activated carbon, VM-COF exhibited much better purification ability. This opens up a new opportunity for the creation of functionalized nanoporous materials for the large-scale purification of waste-generated biogases to address the challenges associated with energy and the environment.

Bimetallic Copper-Cerium-Based Metal-Organic Frameworks for Selective Carbon Dioxide Capture.

Metal-organic frameworks (MOFs) are highly regarded as valuable adsorbent materials in materials science, particularly in the field of CO2 capture. While numerous single-metal-based MOFs have demonstrated exceptional CO2 adsorption capabilities, recent advancements have explored the potential of bimetallic MOFs for enhanced performance. In this study, a CuCe-BTC MOF was synthesized through a straightforward hydrothermal method, and its improved properties, such as high surface area, smaller pore size, and larger pore volume, were compared with those of the bare Ce-BTC. The impact of the Cu/Ce ratio (1:4, 1:2, 1:1, and 3:2) was systematically investigated to understand how adding a second metal influences the CO2 adsorption performance of the Ce-BTC MOF. Various characterization techniques, including scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and N2 BET surface area analysis, were employed to assess the physical and chemical properties of the bare Ce-BTC and CuCe-BTC samples. Notably, CuCe-BTC-1:2 exhibited superior surface area (133 m2 g-1), small pore size (3.3 nm), and large pore volume (0.14 cm3 g-1) compared to the monometallic Ce-BTC. Furthermore, CuCe-BTC-1:2 demonstrated a superior CO2 adsorption capacity (0.74 mmol g-1), long-term stability, and good CO2/N2 selectivity. This research provides valuable insights into the design of metal-BTC frameworks and elucidates how introducing a second metal enhances the properties of the monometallic Ce-BTC-MOF, leading to improved CO2 capture performance.

Surface Electronic Properties-Driven Electrocatalytic Nitrogen Reduction on Metal-Conjugated Porphyrin 2D-MOFs.

Two-dimensional (2D) metal organic framework (MOF) or metalloporphyrin nanosheets with a stable metal-N4 complex unit present the metal as a single-atom catalyst dispersed in the 2D porphyrin framework. First-principles calculations on the 3d-transition metals in M-TCPP are investigated in this study for their surface-dependent electronic properties including work function and d-band center. Crystal orbital Hamiltonian population (-pCOHP) analysis highlights a higher contribution of the bonding state in the M-N bond and antibonding state in the N-N bond to be essential for N-N bond activation. A linear relationship between ΔGmax and surface electronic properties, N-N bond strength, and Bader charge has been found to influence the rate-determining potential for nitrogen reduction reaction (NRR) in M-TCPP MOFs. 2D Ti-TCPP MOF, with a kinetic energy barrier of 1.43 eV in the final protonation step of enzymatic NRR, shows exclusive NRR selectivity over competing hydrogen reduction (HER) and nitrogenous compounds (NO and NO2). Thus, Ti-TCPP MOF with an NRR limiting potential of -0.35 V in water solvent is proposed as an attractive candidate for electrocatalytic NRR.

Robust Zeolitic Tetrazole Framework for Electrocatalytic Dopamine Detection with High Selectivity.

A novel zeolitic tetrazolate framework (ZTF-8) has been synthesized by solvent-free heat-assisted (70 °C) mechanochemical grinding of zinc acetate and 5-methyl tetrazole in the presence of NaOH powder. The structure of ZTF-8 adopts the zeolitic sodalite (SOD) topology with uncoordinated N-heteroatom sites and resembles the structure of the well-known zeolitic imidazole framework ZIF-8. ZTF-8 is exceptionally stable in 0.1 M aqueous acid and base solutions for 60 days at 25 °C. The unique structure with uncoordinated N-heteroatom active sites and exceptional stability of ZTF-8 facilitated the electrocatalytic oxidation of dopamine to dopamine quinone at neutral pH. Without any postsynthetic modification, ZTF-8 is directly used for the facile electrochemical detection of dopamine over a wide range of concentrations (5-550 µM) with a high sensitivity (2410.8 µA mM-1 cm-2). It also demonstrated promising selectivity over other interferents of similar oxidation potential, such as ascorbic acid and uric acid. The DFT study revealed that the ZTF-8 framework has a higher binding energy (-145.07 kJ/mol) and stronger interaction with dopamine than its isostructural ZIF-8 structure (-130.42 kJ/mol).

Accelerating the prediction of CO2 capture at low partial pressures in metal-organic frameworks using new machine learning descriptors.

Metal-Organic frameworks (MOFs) have been considered for various gas storage and separation applications. Theoretically, there are an infinite number of MOFs that can be created; however, a finite amount of resources are available to evaluate each one. Computational methods can be adapted to expedite the process of evaluation. In the context of CO2 capture, this paper investigates the method of screening MOFs using machine learning trained on molecular simulation data. New descriptors are introduced to aid this process. Using all descriptors, it is shown that machine learning can predict the CO2 adsorption, with an R2 of above 0.9. The introduced Effective Point Charge (EPoCh) descriptors, which assign values to frameworks' partial charges based on the expected CO2 uptake of an equivalent point charge in isolation, are shown to be the second most important group of descriptors, behind the Henry coefficient. Furthermore, the EPoCh descriptors are hundreds of thousands of times faster to obtain compared with the Henry coefficient, and they achieve similar results when identifying top candidates for CO2 capture using pseudo-classification predictions.

Metal-Organic Framework-Derived Mesoporous B-Doped CoO/Co@N-Doped Carbon Hybrid 3D Heterostructured Interfaces with Modulated Cobalt Oxidation States for Alkaline Water Splitting.

Heteroatom-doped transition metal-oxides of high oxygen evolution reaction (OER) activities interfaced with metals of low hydrogen adsorption energy barrier for efficient hydrogen evolution reaction (HER) when uniformly embedded in a conductive nitrogen-doped carbon (NC) matrix, can mitigate the low-conductivity and high-agglomeration of metal-nanoparticles in carbon matrix and enhances their bifunctional activities. Thus, a 3D mesoporous heterostructure of boron (B)-doped cobalt-oxide/cobalt-metal nanohybrids embedded in NC and grown on a Ni foam substrate (B-CoO/Co@NC/NF) is developed as a binder-free bifunctional electrocatalyst for alkaline water-splitting via a post-synthetic modification of the metal-organic framework and subsequent annealing in different Ar/H2 gas ratios. B-CoO/Co@NC/NF prepared using 10% H2 gas (B-CoO/Co@NC/NF [10% H2 ]) shows the lowest HER overpotential (196 mV) and B-CoO/Co@NC/NF (Ar), developed in Ar, shows an OER overpotential of 307 mV at 10 mA cm-2 with excellent long-term durability for 100 h. The best anode and cathode electrocatalyst-based electrolyzer (B-CoO/Co@NC/NF (Ar)(+)//B-CoO/Co@NC/NF (10% H2 )(-)) generates a current density of 10 mA cm-2 with only 1.62 V with long-term stability. Further, density functional theory investigations demonstrate the effect of B-doping on electronic structure and reaction mechanism of the electrocatalysts for optimal interaction with reaction intermediates for efficient alkaline water-splitting which corroborates the experimental results.

Acoustomicrofluidic Defect Engineering and Ligand Exchange in ZIF-8 Metal-Organic Frameworks.

A way through which the properties of metal-organic frameworks (MOFs) can be tuned is by engineering defects into the crystal structure. Given its intrinsic stability and rigidity, however, it is difficult to introduce defects into zeolitic imidazolate frameworks (ZIFs)-and ZIF-8, in particular-without compromising crystal integrity. In this work, it is shown that the acoustic radiation pressure as well as the hydrodynamic stresses arising from the oscillatory flow generated by coupling high frequency (MHz-order) hybrid surface and bulk acoustic waves into a suspension of ZIF-8 crystals in a liquid pressure transmitting medium is capable of driving permanent structural changes in their crystal lattice structure. Over time, the enhancement in the diffusive transport of guest molecules into the material's pores as a consequence is shown to lead to expansion of the pore framework, and subsequently, the creation of dangling-linker and missing-linker defects, therefore offering the possibility of tuning the type and extent of defects engineered into the MOF through the acoustic exposure time. Additionally, the practical utility of the technology is demonstrated for one-pot, simultaneous solvent-assisted ligand exchange under ambient conditions, for sub-micron-dimension ZIF-8 crystals and relatively large ligands-more specifically 2-aminobenzimidazole-without compromising the framework porosity or overall crystal structure.

High-Alkaline Water-Splitting Activity of Mesoporous 3D Heterostructures: An Amorphous-Shell@Crystalline-Core Nano-Assembly of Co-Ni-Phosphate Ultrathin-Nanosheets and V- Doped Cobalt-Nitride Nanowires.

Introducing amorphous and ultrathin nanosheets of transition bimetal phosphate arrays that are highly active in the oxygen evolution reaction (OER) as shells over an electronically modulated crystalline core with low hydrogen absorption energy for an excellent hydrogen evolution reaction (HER) can boost the sluggish kinetics of the OER and HER in alkaline electrolytes. Therefore, in this study, ultrathin and amorphous cobalt-nickel-phosphate (CoNiPOx ) nanosheet arrays are deposited over vanadium (V)-doped cobalt-nitride (V3% -Co4 N) crystalline core nanowires to obtain amorphous-shell@crystalline-core mesoporous 3D-heterostructures (CoNiPOx @V-Co4 N/NF) as bifunctional electrocatalysts. The optimized electrocatalyst shows extremely low HER and OER overpotentials of 53 and 270 mV at 10 mA cm-2 , respectively. The CoNiPOx @V3% -Co4 N/NF (+/-) electrolyzer utilizing the electrocatalyst as both anode and cathode demonstrates remarkable overall water-splitting activity, requiring a cell potential of only 1.52 V at 10 mA cm-2 , 30 mV lower than that of the RuO2 /NF (+)/20%-Pt/C/NF (-) electrolyzer. Such impressive bifunctional activities can be attributed to abundant active sites, adjusted electronic structure, lower charge-transfer resistance, enhanced electrochemically active surface area (ECSA), and surface- and volume-confined electrocatalysis resulting from the synergistic effects of the crystalline V3% -Co4 N core and amorphous CoNiPOx shells boosting water splitting in alkaline media.

Three-in-One C2 H2 -Selectivity-Guided Adsorptive Separation across an Isoreticular Family of Cationic Square-Lattice MOFs.

Energy-efficient selective physisorption driven C2 H2 separation from industrial C2-C1 impurities such as C2 H4 , CO2 and CH4 is of great importance in the purification of downstream commodity chemicals. We address this challenge employing a series of isoreticular cationic metal-organic frameworks, namely iMOF-nC (n=5, 6, 7). All three square lattice topology MOFs registered higher C2 H2 uptakes versus the competing C2-C1 gases (C2 H4 , CO2 and CH4 ). Dynamic column breakthrough experiments on the best-performing iMOF-6C revealed the first three-in-one C2 H2 adsorption selectivity guided separation of C2 H2 from 1:1 C2 H2 /CO2 , C2 H2 /C2 H4 and C2 H2 /CH4 mixtures. Density functional theory calculations critically examined the C2 H2 selective interactions in iMOF-6C. Thanks to the abundance of square lattice topology MOFs, this study introduces a crystal engineering blueprint for designing C2 H2 -selective layered metal-organic physisorbents, previously unreported in cationic frameworks.

Hetero-metallic metal-organic frameworks for room-temperature NO2 sensing.

Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size has been considered a promising candidate for developing room temperature highly-sensitive gas sensors. In comparison, the hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. In this work, the Fe atoms in the Fe3 (Porous coordination network) PCN-250 MOFs are partially replaced by transition metal Co, Mn, and Zn through a facile hydrothermal approach, leading to the formation of hetero-metallic MOFs (Fe2IIIMII, M = Co, Mn, and Zn). While the PCN-250 framework is maintained, the morphological and electronic band structural properties are manipulated upon the partial metal replacement of Fe. More importantly, the room temperature NO2 sensing performances are significantly varied, in which Fe2Mn PCN-250 demonstrates the largest response magnitude for ppb-level NO2 gas compared to those of pure Fe3 PCN-250 and other hetero-metallic MOF structures mainly attributed to the highest binding energy of NO2 gas. This work demonstrates the strong potential of hetero-metallic MOFs with carefully engineered substituted metal clusters for power-saving and high-performance gas sensing applications.

Prediction of O2/N2 Selectivity in Metal-Organic Frameworks via High-Throughput Computational Screening and Machine Learning.

Machine learning (ML), which is becoming an increasingly popular tool in various scientific fields, also shows the potential to aid in the screening of materials for diverse applications. In this study, the computation-ready experimental (CoRE) metal-organic framework (MOF) data set for which the O2 and N2 uptakes, self-diffusivities, and Henry's constants were calculated was used to fit the ML models. The obtained models were subsequently employed to predict such properties for a hypothetical MOF (hMOF) data set and to identify structures having a high O2/N2 selectivity at room temperature. The performance of the model on known entries indicated that it would serve as a useful tool for the prediction of MOF characteristics with r2 correlations between the true and predicted values typically falling between 0.7 and 0.8. The use of different descriptor groups (geometric, atom type, and chemical) was studied; the inclusion of all descriptor groups yielded the best overall results. Only a small number of entries surpassed the performance of those in the CoRE MOF set; however, the use of ML was able to present the structure-property relationship and to identity the top performing hMOFs for O2/N2 separation based on the adsorption and diffusion selectivity.

Elucidating the mechanisms of Paraffin-Olefin separations using nanoporous adsorbents: An overview.

Light olefins are the precursors of all modern-day plastics. Olefin is always mixed with paraffins in the time of production, and therefore it needs to be separated from paraffins to produce polymer-grade olefin. The state-of-the-art separation technique, cryogenic distillation, is highly expensive and hazardous. Adsorption could be a novel, sustainable, and inexpensive separation strategy, provided a suitable adsorbent can be designed. There are different types of mechanisms that were harnessed for the separation of olefins by adsorption, and in this review, we have focused our discussion on those mechanisms. These mechanisms include, (a) Affinity-based separation, like pi complexation and hydrogen bonding, (b) Separation based on pore size and shape, like size-exclusion and gate-opening effect, and (c) Non-equilibrium separation, like kinetic separation. In this review, we have elaborated each of the separation strategies from the fundamental level and explained their roles in the separation processes of different types of paraffins and olefins.

Reversing Benzene/Cyclohexane Selectivity through Varying Supramolecular Interactions Using Aliphatic, Isoreticular MOFs.

Effective solid-state adsorbent materials, such as metal organic frameworks (MOFs), rely upon tailored void spaces for selective adsorption of one component from a mixture. This is particularly crucial when separating challenging mixtures such as benzene (Bz) and cyclohexane (Cy) requiring a highly expensive and energy intensive process. Employing bulky "3D-linkers" to construct MOFs leads to materials with unique, contoured pore shapes which consequently allow for significant control over guest adsorption preferences. Investigation into these selectivity preferences is key to identifying suitable materials for industrial separations and is an area currently underexplored. Here, we provide an in-depth investigation exploring the selectivity path between planar and 3D-linkers and their preference to adsorb either Cy or Bz. To validate this principle, the adsorption selectivity of Cy and Bz in 3DL-MOF-1 ([Zn4O(pdc)3] (pdc = bicylo[1.1.1]pentane-1,3-dicarboxylate), CUB-5, and MOF-5 was explored. MOF-5 exhibits a selective preference for Cy adsorption at low pressures, contrary to popular belief, while CUB-5 and 3DL-MOF-1 are Bz selective. DFT-D3 calculations and breakthrough simulations explore this behavior and highlight CUB-5 and MOF-5 as strong candidates for future separation materials.

Selective capture of carbon dioxide from hydrocarbons using a metal-organic framework.

Efficient and sustainable methods for carbon dioxide capture are highly sought after. Mature technologies involve chemical reactions that absorb CO2, but they have many drawbacks. Energy-efficient alternatives may be realised by porous physisorbents with void spaces that are complementary in size and electrostatic potential to molecular CO2. Here, we present a robust, recyclable and inexpensive adsorbent termed MUF-16. This metal-organic framework captures CO2 with a high affinity in its one-dimensional channels, as determined by adsorption isotherms, X-ray crystallography and density-functional theory calculations. Its low affinity for other competing gases delivers high selectivity for the adsorption of CO2 over methane, acetylene, ethylene, ethane, propylene and propane. For equimolar mixtures of CO2/CH4 and CO2/C2H2, the selectivity is 6690 and 510, respectively. Breakthrough gas separations under dynamic conditions benefit from short time lags in the elution of the weakly-adsorbed component to deliver high-purity hydrocarbon products, including pure methane and acetylene.

Plasmonic metal-organic framework nanocomposites enabled by degenerately doped molybdenum oxides.

Metal-organic frameworks (MOFs) nanocomposites are under the limelight due to their unique electronic, optical, and surface properties for various applications. Plasmonic MOFs enabled by noble metal nanostructures are an emerging class of MOF nanocomposites with efficient solar light-harvesting capability. However, major concerns such as poor photostability, sophisticated synthesis processes, and high fabrication cost are raised. Here, we develop a novel plasmonic MOF nanocomposite consisting of the ultra-thin degenerately doped molybdenum oxide core and the flexible iron MOF (FeMOF) shell through a hydrothermal growth, featuring low cost, facile synthesis, and non-toxicity. More importantly, the incorporation of plasmonic oxides in the highly porous MOF structure enhances the visible light absorbability, demonstrating improved photobleaching performances of various azo and non-azo dyes compared to that of pure FeMOF without the incorporation of oxidative agents. Furthermore, the nanocomposite exhibits enhanced sensitivity and selectivity towards NO2 gas at room temperature, attributed to the electron-rich surface of plasmonic oxides. This work possibly broadens the exploration of plasmonic MOF nanocomposites for practical and efficient solar energy harvesting, environmental remediation, and environmental monitoring applications.

Mixed donor, phenanthroline photoactive MOFs with favourable CO2 selectivity.

Mixed donor phenanthroline-carboxylate linkers were combined with MnII or ZnII to form photoactive MOFs with large pore apertures. The MOFs display high CO2 adsorption capacities, which consequently causes structural framework flexibility, and align with favorable metrics for selective CO2 capture. The photophysical properties of the MOFs were investigated, with the MnII MOF giving rise to short triplet LMCT lifetimes.

A Water-Stable Cationic Metal-Organic Framework with Hydrophobic Pore Surfaces as an Efficient Scavenger of Oxo-Anion Pollutants from Water.

Water contamination due to heavy metal-based toxic oxo-anions (such as CrO42- and TcO4-) is a critical environmental concern that demands immediate mitigation. Herein, we present an effort to counter this issue by a novel chemically stable cationic metal-organic framework (iMOF-2C) with strategic utilization of a ligand with hydrophobic core, known to facilitate such oxo-anion capture process. Moreover, the compound exhibited very fast sieving kinetics for such oxo-anions and a very high uptake capacity for CrO42- (476.3 mg g-1) and ReO4- (691 mg g-1), while the latter being employed as a surrogate analogue for radioactive TcO4- anions. Notably, the compound showed excellent selectivity even in the presence of other competing anions such as NO3-, Cl-, SO42-, ClO4-. etc.. Furthermore, the compound possesses excellent reusability (up to 10 cycles) and is also employed to a stationary phase ion column to decontaminate the aforementioned oxo-anions from water.

The Effect of Sterically Active Ligand Substituents on Gas Adsorption within a Family of 3D Zn-Based Coordination Polymers.

An investigation of the adsorption properties of two structurally related, 3D coordination polymers of composition Zn(2-Mehba) and Zn(2,6-Me2hba) (2-Mehba = the dianion of 2-methyl-4-hydroxybenzoic acid and 2,6-Me2hba = the dianion of 2,6-dimethyl-4-hydroxybenzoic acid) is presented. A common feature of these structures are parallel channels that are able to accommodate appropriately sized guest molecules. The structures differ with respect to the steric congestion within the channels arising from methyl groups appended to the bridging ligands of the network. The host network, Zn(2-Mehba), is able to take up appreciable quantities of H2 (77 K) and CO2 and CH4 (298 K) in a reversible manner. In regard to the adsorption of N2 by Zn(2-Mehba), there appears to be an unusual temperature dependence for the uptake of the gas such that when the temperature is increased from 77 to 298 K the uptake of N2 increases. The relatively narrow channels of Zn(2,6-Me2hba) are too small to allow the uptake of N2 and CH4, but H2 molecules can be adsorbed. A pronounced step at elevated pressures in CO2 and N2O isotherms for Zn(2,6-Me2hba) is noted. Calculations indicate that rotation of phenolate rings leads to a change in the available intraframework space during CO2 dosing.

Metal-Organic Framework-Enhanced Solid-Phase Microextraction Mass Spectrometry for the Direct and Rapid Detection of Perfluorooctanoic Acid in Environmental Water Samples.

We report the development of metal-organic framework (MOF)-based probes for the direct and rapid detection and quantification of perfluorooctanoic acid (PFOA) by mass spectrometry. Four water-resistant MOFs-ZIF-8, UiO-66, MIL88-A, and Tb2(BDC)3-were coated on poly(dopamine) precoated stainless steel needles and used to rapidly preconcentrate PFOA from water for direct analysis by nanoelectrospray ionization mass spectrometry. The analytical performance of each MOF for detecting PFOA was correlated with both the calculated binding energy of the MOF for PFOA and the relative change in the surface area of the MOF upon exposure to PFOA. MOF-functionalized probes can be used for the rapid (<5 min) and sensitive quantification of PFOA molecules at low ng L-1 levels in environmental water samples (i.e., tap water, rainwater, and seawater) with no sample preparation. The limit of detection of PFOA in ultrapure water was 11.0 ng L-1. Comparable accuracy to an accredited analytical method was achieved, despite the MOF-functionalized probe approach being ∼40 times quicker and requiring ∼10 times less sample. These features indicate that MOF-coated probes are promising for the direct and rapid monitoring of polyfluorinated substances and other pollutants in the field.

Enhancing Multicomponent Metal-Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations.

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal-organic frameworks (MOFs) that contain the three-dimensional cubane-1,4-dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB-30), comprising cdc, 4,4'-biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π-π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.